The goal of the proposed studies is to understand the functional organization of layers 2/3 in visual cortex of cats at the level of single neurons. Functional architecture of visual cortex has been studied with two approaches: (1) electrophysiology of single neurons, and (2) optical imaging, which provides functional maps at low resolution: >100 mu/m. The new technique of calcium imaging with two-photon laser-scanning microscopy (resolution <1 mu/m) bridges the gap between single-unit electrophysiology and conventional optical imaging. Using this technique, we will create the first maps of visual cortical function at the scale of single neurons. We call cortical organization at this level functional micro-architecture. We will examine functional micro-architecture with a three-pronged strategy: intrinsic-signal imaging, two-photon imaging, and electrophysiology. We will first obtain maps of visual cortex with conventional optical imaging, then zoom in with two-photon microscopy to examine smaller regions at single-cell resolution. In Aim 1, we will examine the fine-scale clustering of neurons according to receptive-field attributes such as orientation selectivity, direction selectivity, ocular dominance and retinotopy. In Aim 2, we will combine single-cell calcium imaging with electrophysiology: to calibrate the calcium signal, to examine co-active ensembles of neurons and (with intracellular stimulation) to determine the influence of one neuron on its potential targets in the circuit.